Photosensitive flexographic printing plates have a layer of a photo-polymer that undergoes a chemical reaction when exposed to actinic radiation. Typically, ultraviolet light is used to expose the photo-polymer in flexographic printing plates. Upon exposure to ultraviolet light the photo-polymer undergoes a cross-linking reaction. The printing plates are exposed through a mask which blocks the actinic radiation from reaching certain portions of the photo-polymer. The mask is patterned with a pattern of opaque and non-opaque areas that form an image to be recreated on the printing plate. After exposure the printing plate can be developed. After development the pattern of exposed and unexposed areas on the printing plate provide a textured surface that may be used to print an image.
Some types of printing plates are used in conjunction with a separate mask. Typically the mask is made from a half-tone film, such as a silver-halide film, which can be placed on top of the plate before the plate is exposed. A problem with this method is that it is necessary to keep the film in accurate alignment and in intimate contact with the plate during exposure. If the mask is misaligned with the printing plate or if the mask is not kept in intimate contact with the printing plate during exposure then the results obtained will be less than optimum. Even when a separate mask is applied carefully to a printing plate the results obtained are often not optimal.
Another method for exposing flexographic plates involves providing a photosensitive plate that has a mask formed integrally with the plate. Such plates include a layer of a photosensitive material, such as a photo-polymer gel, underlying a layer of mask material. Because the mask is integral with the printing plate, this approach provides a mask which is always in intimate contact with the photo-polymer layer and which is always properly aligned with the printing plate. A problem with this method is that the material from which the mask layer is made can diffuse into the photo-polymer gel. Over time, this can degrade the gel. As a result, printing plates that include an integral mask may have undesirably short shelf lives.
The problem of low shelf life is exacerbated because there are a large number of different types of photo-polymers used in printing plates. All have different characteristics and are used in different applications. If each of the different types of printing plates has a short shelf life then it is often not practical for a user to keep on hand a stock of printing plates of any types that will not be quickly used up.
A further problem with printing plates that have integral masking layers is that the masking layer on existing plates is typically fragile. A scratch in any portion of the masking layer can make the entire plate unusable.
U.S. Pat. No. 5,262,275 issued to Fan discloses a flexographic printing element that incorporates a barrier layer that separates an infrared-ablatable masking layer and a photo-polymerizable layer. The barrier layer prevents migration of material from the masking layer into the photo-polymerizable layer. While this construction improves the shelf life of a printing plate it does not avoid the requirement to maintain a number of different types of printing plate in stock that have different types of photo-polymer layer. It also does not address the problem that the masking layer of a printing plate may become damaged, thereby making the plate unusable.
U.S. Pat. No. 6,180,325 issued to Gelbart discloses methods for exposing photosensitive printing plates that address some of the limitations of the prior art. The methods involve mounting a printing plate to a holder; applying a patterned coating to an outer surface of the printing plate by moving a coating applicator relative to the holder; and, exposing the printing plate through the patterned coating to actinic radiation without removing the printing plate from the holder. One embodiment of the Gelbart system applies a mask using an inkjet printhead which ejects ink or other opaque material. The properties of the inkjetted mask material are not specified.
There remains a need in industry for a high-speed process of creating a flexographic mask directly on a photo-imageable flexographic plate material.